The AppleTalk local area network described by Sidhu et al. in U.S. Pat. No. 4,689,786 is a cost effective protocol for the exchange of data among a large variety of data processing devices through a simplex communications medium. The most common embodiment of a physical communications channel for AppleTalk is a passive, shielded, twisted pair cable that is routed from node to node. There are several deficiencies inherent in the use of shielded cable and a bus topology.
Foremost among them is the difficulty and expense of routing cables from point to point, especially within occupied rooms or buildings. Second, shielded cable is more expensive and less common than unshielded cable, and it has higher transmission loss characteristics. Third, a passive medium such as shielded cable contains no means for signal regeneration, and this limits the number of devices and amount of cable that can be interconnected. Finally, a passive medium contains no means to isolate and contain the deleterious effects of common faults such as short or open circuits or nodes that fail to obey the media access protocols. As disclosed in copending application Ser. No. 06/899,413, filed Aug. 22, 1986 by Reese Jones and assigned to Farallon Computing, Inc., the assignee of this application, these deficiencies can be overcome in a broadcast communications system based on unshielded telephone quality cabling arranged in a hierarchical star topology with active signal regenerators at the star hubs.
Telephone wiring is inexpensive, abundant, and already installed in the majority of sites that might require data communications. Telephone wiring most often uses a hierarchical star topology, such as illustrated in FIG. 1. Separate cable are routed from each point of service to a common point of interconnection, such as a wiring closet. These interconnection points are wired back to another point of interconnection, such as a distribution frame servicing an entire building. To build a broadcast communications channel using a hierarchical star wiring system, one must provide active multiport signal regenerators at the star centers (e.g., in the wiring closets).
A multiport signal regenerator must attach to a number of wire pairs routed to points of potential data service consumption. Data processing equipment may or may not be attached to any particular access point. To initiate communications with another device, a node transmits a message onto a wire pair to which it is attached. Because each wire pair is a multiaccess simplex medium, the message travels in both directions and arrives at all other nodes attached to the same pair and to the signal regenerator. The basic function of the signal regenerator is to sense the presence of an incoming message, regenerate it, and transmit it to all other wire pairs. Full interconnectivity is achieved exactly as on a passive bus; that is, every node can receive the messages of every other node. A signal regenerator may connect to other signal regenerators as necessary to interconnect as many access points as desired.
There are several aspects of the data link protocols of AppleTalk that make the design of a multiport signal regenerator difficult. A specification of minimum interpacket delay has been omitted. It is therefore possible for the beginning of one transmission to overlap or immediately follow the end of another. The AppleTalk protocols also do not specify an interval at the start of each transmission for the purpose of synchronizing bit timing recovery circuits. These two omissions require that a signal generator be able to very quickly release one port and select another as messages are relayed. If making port selection takes too long, the beginning portions of messages can be lost or corrupted.
An additional problem with the AppleTalk data link protocols is the inclusion of a fast "sync" pulse at the start of certain transmissions. The purpose of this pulse is to decrease the probability of collisions of signals issued by stations wishing to transmit. Unfortunately, this sync pulse has the same characteristics as impulse noises; thus this pulse might otherwise be removed by the system as undesirable. However, because these sync pulses must be faithfully reproduced, a signal generator must quickly select and release ports.
The AppleTalk data link protocols assume a multiaccess, simplex transmission medium. The most common implementation of a line is a twisted wire pair to which nodes and signal regenerators couple through an isolation transformer. Data links such as the AT & T Starlan network generally use two lines, each a pair of wires, connected between each computer or computer peripheral device (collectively designated as "computer products" herein) and a central control device; one line serves to receive messages and the other line serves to transmit messages. Here, line driver and receiver circuits are coupled to the same line, one pair of wires over which all messages travel to and from a given computer product. By increasing driver output levels and increasing receiver sensitivity, one can increase the amount of cable through which error-free transmission can be made. However, doing so increases the time it takes a driven line and coupling transformer to settle after relaying a message. Reflections from shorted or unterminated lines also increase the time it takes a driven line to settle. If these transient conditions are mistaken for a valid incoming signal, a port could be erroneously selected. Further, a signal regenerator could become unstable, continuously selecting and deselecting ports falsely, or repeatedly ignoring valid incoming signals.
Thus, fast response time, noise immunity, and sensitivity require a three way tradeoff for the design of a multiport AppleTalk signal regenerator.